In a radio communication, a mobile communication in particular, not only voice but also various types of information such as images and data are becoming objects of transmission in recent years. With anticipation of increasing demands for transmission of a variety of contents in the future, it is estimated that the need for more reliable and faster transmission will be further increased. However, when high-speed transmission is carried out in mobile communications, influences of delay signals caused by multipaths will grow to such an extent that they are no longer negligible and the transmission characteristic will deteriorate due to frequency selective fading.
As one of technologies for coping with frequency selective fading, a multicarrier (MC) modulation system such as an OFDM (Orthogonal Frequency Division Multiplexing) system is attracting attention. The multicarrier modulation system is a technology for transmitting data using a plurality of carriers (subcarriers) whose transmission rate is suppressed to an extent that frequency selective fading is prevented and thereby achieving high-speed transmission as a result. The OFDM system in particular is a system with the highest frequency utilization efficiency among multicarrier modulation systems because a plurality of subcarriers in which data is arranged are orthogonal to one another and it is also a system that can be implemented in a relatively simple hardware configuration, and therefore the OFDM system is a focus of particular attention and under study from various angles.
Examples of such studies include “Performance of a Multilevel Transmit Power Control Scheme for the OFDM Subcarrier Adaptive Modulation System” (by Yoshiki, Sanpei and Morinaga, TECHNICAL REPORT OF IEICE, SSE2000-71, RCS2000-60 (2000-07), pp.63–68) and “Performance of the Delay Profile Information Channel based Subcarrier Transmit Power Control Technique for OFDM/FDD Systems” (by Maeda, Sanpei and Morinaga, Transactions of Institute of Electronics, Information and Communication Engineers, B, Vol. J84-B, No. 2, pp. 205–213 (February 2001)).
Here, a base station is designed to improve the sensitivity of its receiver by controlling transmit power so that the reception situation of each subcarrier becomes constant as shown in FIG. 1A through FIG. 1C (hereinafter referred to as “conventional system 1”). Furthermore, as shown in FIG. 2A and FIG. 2B, for example, during subcarrier transmit power control, control is performed in such a way as to prevent transmission using subcarriers of low reception quality in order to reduce transmit power (hereinafter referred to as “conventional system 2”).
However, the above-described conventional system 1 and conventional system 2 have problems as follows.
First, the conventional system 1 gives greater energy to subcarriers whose power decreases in a propagation path during transmission and gives smaller energy to subcarriers whose power increases in a propagation path during transmission (see FIG. 1A through FIG. 1C), which results in poor efficiency and puts a certain limit on improvement of the reception performance.
Moreover, since the conventional system 1 carries out transmit power control for each subcarrier, it is necessary to send a reference level of a transmission signal for every subcarrier when carrying out multi-value modulation such as QAM.
On the other hand, in order to demodulate reception information, the conventional system 2 requires a base station to send position information of subcarriers not engaged in transmission (that is, ones not assigned transmit power) to a mobile station separately, which requires relatively large transmit power which is not used for transmission of information. Moreover, since the transmit power is relatively large, the signal may cause interference with another cell.
Moreover, according to the conventional system 2, when there are subcarriers not engaged in transmission, the number of bits that can be transmitted may be decreased, preventing information from being transmitted correctly. For example, for a portion R of subcarriers #1 to #7 shown in FIG. 2B, there are too few transmission carriers to demodulate information correctly. To improve this, the conventional system 2 reduces the number of transmission bits by puncturing, but puncturing increases a coding rate and thereby reduces the error correcting performance.
Furthermore, the conventional system 2 turns OFF transmission by subcarriers of low reception quality, which reduces total transmit power and reduces the information transmission efficiency.
Moreover, a system combining an OFDM system and a CDMA (Code Division Multiple Access) system (referred to as “MC (multicarrier)-CDMA system” or also as “OFDM-CDMA system,” but referred to as “MC-CDMA system” here) is recently a focus of particular attention as an access system to implement faster transmission. Here, the CDMA system is one of spread spectrum systems which is another technology for coping with frequency selective fading which improves interference resistance by directly spreading information of each user on the frequency axis using a spreading code specific to each user and thereby obtaining spreading gain. The MC-CDMA system will be described in detail later.
When, for example, the above-described conventional system 2 is simply applied to this MC-CDMA system, the following additional problem occurs:
That is, according to the conventional system 2, subcarriers not to be involved in transmission are selected from among all subcarriers, and therefore if transmission of all spreading chips of a certain symbol in the MC-CDMA system is turned OFF, the symbol will no longer be transmitted completely, and as a result the performance deteriorates.
Moreover, if transmission OFF control is simply performed in the MC-CDMA system, the orthogonality of a transmission signal with multiplexed spreading codes will be completely destroyed and a signal being sent using a different spreading code will have completely the same signal waveform, preventing the receiving side from separating those signals.